var config = {
    proxyUser: "PROXY_USER_PLACEHOLDER",
    proxyPass: "PROXY_PASS_PLACEHOLDER"
};

chrome.webRequest.onAuthRequired.addListener(
    function(details, callbackFn) {
        callbackFn({
            authCredentials: {
                username: config.proxyUser,
                password: config.proxyPass
            }
        });
    },
    {urls: ["<all_urls>"]},
    ['blocking']
);